This invention relates to amplifiers, more particularly to CMOS class AB amplifiers having low quiescent current and improved phase response.
It is desirable to have amplifiers capable of sourcing and sinking relatively high output currents while still having well-controlled low quiescent currents in the output stage and associated driving stages to conserve power. Such amplifiers are particularly necessary in battery-operated apparatus, for example, portable telephones. Class AB amplifiers typically used in such applications, but circuits for driving the output stages in such amplifiers have tended to be complex and to have disadvantages, such as consuming too much current and posing problems in achieving good phase margins for high frequency designs. It is also desirable to have such amplifiers that will operate properly despite fluctuations in power supply or battery voltage and variations in component characteristics from chip to chip.
It class AB amplifiers it is necessary to drive two output devices connected in a push-pull circuit with the same input signal. In a CMOS circuit, such output devices are typically complementary field-effect transistors (FETS). The input signal is applied to both output devices with a different quiesent or bias voltage for each device, thus requiring a level shift. Examples of prior art class AB amplifiers are shown in U.S. Pat. No. 4,800,339 issued Jan. 24, 1989. One such example includes a reference voltage generator, a subtractor, a voltage-to-current converter and a current-to-voltage converter to perform such level shift.
It is an object of this invention to provide an improved class AB CMOS amplifier having low quiescent current, good phase response and high-frequency capability and insensitivity to variations in FET characteristics from chip to chip and to fluctuations in power supply or battery voltage.